Emergent ferroelectricity in the two-dimensional Janus MoSSe monolayer driven by nondegenerate phonon instability

We report the discovery of bistable polar states with switchable polarization in the Janus monolayer 1T-MoSSe, induced by symmetry breaking in its chalcogen atomic layers. Our results demonstrate that Janus 1T-MoSSe exhibits two out-of-plane bistable polar states with switchable polarization, rather than polarization emerging from a non-polar phase, which represents an unconventional form of ferroelectric-like behavior. First-principles calculations and phenomenological modeling reveal that the inequivalent stacking of sulfur and selenium (S/Se) atoms breaks central inversion symmetry, activating non-degenerate phonon modes at the K-point (K₂/K₃) that drive the structural transformation between metastable d1T_S and d1T_Se phases. This coupling enables bipolar control of out-of-plane polarization through atomic displacements and charge redistribution, resulting in a polarization change of ΔP ≈ ± 0.3 μC/cm². The Landau free energy analysis indicates that anharmonic terms and inter-mode coupling generate an asymmetric double-well potential, which is essential for the stabilization of bistable polar states. Molecular dynamics simulations show that the d1T_S phase remains stable at high temperatures, whereas the d1T_Se phase undergoes an irreversible phase transition near 300 K, accompanied by a Peierls-like distortion of the Mo atomic chain. This transition is driven by differences in electronegativity, atomic radius, and d–p orbital hybridization between S and Se. Our findings establish a theoretical framework for engineering nonlinear responses in two-dimensional (2D) ferroelectrics and suggest that low-energy polarization reversal at room temperature can be achieved through strain or electric-field control, offering promising opportunities for non-volatile memory and piezoelectric sensing applications.